Characterization of humic-rich hydrocolloids and their metal species by means of competing ligand and metal exchange - an on-site approach

Abstract

An improved on-site characterization of humic-rich hydrocolloids and their metal species in aquatic environments was the goal of the present approach. Both ligand exchange with extreme chelators ( diethylenetetraaminepentaacetic acid ( DTPA), ethylendiaminetetraacetic acid ( EDTA)) and metal exchange with strongly competitive cations (Cu(II)) were used on-site to characterize the conditional stability and availability of colloidal metal species in a humic-rich German bogwater lake ( Venner Moor, Munsterland). A mobile time-controlled tangential-flow ultrafiltration technique (cut-off: 1 kDa) was applied to differentiate operationally between colloidal metal species and free metal ions, respectively. DOC ( dissolved organic carbon) and metal determinations were carried out off-site using a home-built carbon analyzer and conventional ICP-OES ( inductively-coupled plasma-optical emission spectrometry), respectively. From the metal exchange equilibria obtained on-site the kinetic and thermodynamic stability of the original metal species ( Fe, Mn, Zn) could be characterized. Conditional exchange constants K ex obtained from aquatic metal species and competitive Cu(II) ions follow the order Mn > Zn >> Fe. Obviously, Mn and Zn bound to humic-rich hydrocolloids are very strongly competed by Cu( II) ions, in contrast to Fe which is scarcely exchangeable. The exchange of aquatic metal species (e.g. Fe) by DTPA/EDTA exhibited relatively slow kinetics but rather high metal availabilities, in contrast to their Cu(II) exchange.